We shall continue research on the molecular organization of chromosomes using mainly electron microscopy, particularly the High Voltage Electron Microscope (HVEM) at the U. of Wisconsin-Madison. Where feasible cytochemical methods including ferritin and peroxidase labelled antibodies will be used to localize particular molecules at the EM-level. Cold stage and hydration stage in the HVEM will be applied to the study of hydrated chromatin. Specific attention will be paid to the arrangement of DNA and histones in the 10 nm chromatin fiber (existence of histone beads in native nuclei), the origin of the native 20 nm chromatin fiber, the changes in the chromatin fiber in active regions engaged in transcription, and the organization of chromomeres (polytene chromosome bands, lampbrush chromosomes). The nature of mitotic condensation including structural aspects of metaphase banding and the structure and function of kinetochores. The relationship of chromatin to the nuclear envelope and details of nuclear pore structure in relation to nuclear-cytoplasmic transport will be investigated.